This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Back in October 2007 (at Tet Zoo ver 2) I wrote a very brief article on a poorly known, gigantic, deeply weird South American frog: the Helmeted water toad, Chilean giant frog or Gay’s frog* Calyptocephalella gayi (long known – incorrectly it turns out – as Caudiverbera caudiverbera). Back in 2007, so little information was available on this frog that just about the only thing I said was that it’s really big, and that it lives in Chile. The fact that Calyptocephalella has a surprisingly good and extensive fossil record came up in the comments. That was about it. Also of note is that available pictures of this remarkable frog were just about absent online during 2007 (and before).
Today, I’m happy to say that things are somewhat different, and that there’s both more information, and far more imagery, concerning this species online. So now is a good time to revisit it. I recently drew a picture of one as well, but that’s coincidental.
* Not Gray’s frog, as it says in some books.
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C. gayi is gigantic, record-holding females having an SVL of 320 mm. They can weigh as much as 1 kg (Castañeda et al. 2006) [photo above by José Grau de Puerto Montt]. This is a hugely robust, broad-bodied, wide-headed frog with heavily ossified skull bones that have a strongly pitted surface texture, those last features explaining one of its common names, of course. While little direct information is available on its biology and behaviour, it seems to be strongly aquatic and associated mostly with deep pools and large lagoons. Adults are reported to eat assorted arthropods but it’s their predation on vertebrates that makes them especially interesting: fish, small birds and mammals are all reported prey items, as are other frogs including other Helmeted water toad. Some sources say that this species is almost a frog-eating specialist among frogs.
Helmeted water toad tadpoles are also huge, reaching 150 mm in cases and exceeding 30 g. Stop for a moment and imagine a tadpole that size. I used to own an American bullfrog Lithobates catesbeianus tadpole, regarded by everyone who ever saw it as gargantuan, and yet it was ‘only’ 110 mm long. 150 mm is just... mind-blowing. Anurans with enormous tadpoles generally (perhaps universally) have a very flexible, variable developmental schedule. Sure enough, Helmeted water toad tadpoles normally take anywhere between 5 and 12 months to reach metamorphosis, and some have been reported to remain tadpoles for an incredible two years (Diaz & Valencia 1985).
C. gayi tadpoles are sluggish and slow-swimming, and are bulky, light green animals with a deep, proportionally short tail that isn’t clearly differentiated from the body (Diaz & Valencia 1985). They feed mostly on diatoms and other benthic algae before becoming more carnivorous closer to metamorphosis (Castañeda et al. 2006).
‘Leptodactylids’ that aren’t and are actually closer to Australasians
Where does the Helmeted water toad fit within anuran phylogeny? The idea that it’s a hyloid – part of the major neobatrachian group that includes treefrogs (Hylidae), toads (Bufonidae), poison-dart frogs (Dendrobatidae), glassfrogs (Centrolenidae) and so on – has been followed for decades on the basis of anatomical, molecular and biogeographical evidence. Standard Operating Procedure when dealing with hyloids that don’t belong to any of those major lineages has been to classify them within Leptodactylidae, a group that never really had a precise definition and has been a classic 'taxonomic wastebasket'.
Recent molecular studies find Calyptocephalella to group together with the Chilean toads (Telmatobufo) (Frost et al. 2006, Pyron & Wiens 2011), both forming a small clade that has to have the delightful name Calyptocephalellidae [adjacent Telmatobufo photo by Edgardo Patricio Flores]. Somewhat surprisingly, this group is not nested within the mass of hyloid lineages that were previously lumped into Leptodactylidae. Instead, it forms a sister-group relationship with Myobatrachidae, the Australasian ground frogs or southern frogs. An extensive discussion of biogeography and hyloid history should follow at this point but I regret I’ll have to leave that to another time.
I’m also going to mostly avoid discussion of Calyptocephalella fossil history here – several fossil species known as far back as the Eocene have been named, and remains from the Paleocene and even Upper Cretaceous have been referred to this frog lineage. At least one of the fossil species (C. parodii from the Miocene, originally named Gigantobatrachus) reached even greater size than the living one, since one specimen has an SVL of c 350 mm. UPDATE: Tim Morris has reminded me of the partial Calyptocephalella humerus described from the Eocene of Chile by Otero et al. (2014). This bone indicates an SVL for its owner of 580-591 mm.
People love to kill frogs
As is so often the case with remarkable frogs, C. gayi may be in trouble and certainly seems to have declined and even disappeared across part of its range. Habitat loss, the introduction of alien trout, and pollution have all been linked to its decline (Veloso et al. 2010), as has extensive harvesting for its apparently succulent flesh. There is in fact so much call for this frog as a food item that there’s interest in raising it commercially (Toledo et al. 2014): this is in its early stages but the ease with which tadpoles can be raised and fed is encouraging (Toledo et al. 2014).
Within recent years, the species has been the subject of an extensive native commercial trade (Días-Páez & Ortiz 2003) and has also been imported in some numbers to the USA. Given that the species has recently started to appear in the herptoculture industry – a worrying trend that often involves unscrupulous collecting – I assume that these North American imports involve the pet trade. But I wonder whether anyone is checking whether all that frog meat sold and consumed in the USA (and elsewhere) is really from the non-endangered species that people think it’s from...
Waitaminute -- didn't I say that I would write about the remaining 'strange bedfellow frogs'? Ooops.
For previous Tet Zoo articles on frogs and toads, see...
In pursuit of Romanian frogs (part II: WESTERN PALAEARCTIC WATER FROGS!!)
The toads series comes to SciAm: because Africa has toads too
North American spadefoot toads and their incredible fast-metamorphosing, polymorphic tadpoles
Frogs you may not have heard of: Brazil’s Cycloramphus ‘button frogs’
‘Strange bedfellow frogs’ (part I): rotund, adorable brevicipitids
Refs - -
Castañeda, L. E., Sabat, P., Gonzalez, S. P., & Nespolo, R. F. 2006. Digestive plasticity in tadpoles of the Chilean giant frog (Caudiverbera caudiverbera): factorial effects of diet and temperature. Physiological and Biochemical Zoology 79, 919-926.
Diaz N. F. & Valencia, J. 1985. Larval morphology and phenetic relationships of the Chilean Alsodes, Telmatobius, Caudiverbera and Insuetophrynus (Anura, Leptodactylidae). Copeia 1985, 175-181.
Otero, R. A., Jimenez-Huidobro, P., Soto-Acuña, S. & Yury-Yáñez, R. E. 2014. Evidence of a giant helmeted frog (Australobatrachia, Calyptocephalellidae) from Eocene levels of the Magallanes Basin, southernmost Chile. Journal of South American Earth Sciences 55, 133-140.
Veloso, A., Formas, R. & Gerson, H. 2010. Calyptocephalella gayi. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 16 January 2015.